1. Field of the Invention
The present invention relates to electronic circuits and, more specifically, to a trim circuit that increases precision in on-chip resistors.
2. Description of the Prior Art
In many applications, a precise value of resistance is necessary on an integrated circuit chip. It is particularly important when the on-chip circuit must interface with external components, such as a circuit on a printed circuit board, a cable connector, or another chip. A classic example of a circuit requiring precise resistance is an on-chip terminator for a transmission line cable. For an on-chip driver to drive and receive properly, it must be terminated with the characteristic impedance of the cable. An imprecise resistance in the terminator could result in undesirable ringing and other transmission line effects.
Typically, on-chip resistors are designed for nominal process and temperature variations. The tolerances obtained by this method frequently causes performance limitations. This is due to the wide value variations that are caused by production and environmental conditions.
Also, in some applications it is desirable to adjust the terminator to match different design values due to a package change or a connector change. Current on-chip resistors are not capable of changing resistance values once the chip has been manufactured.
Therefore, there is a need for an on-chip resistor that may be tuned to a precise resistance after the chip on which the resistor resides has been manufactured.
The disadvantages of the prior art are overcome by the present invention which, in one aspect, is a resistance circuit that includes an off-chip resistor having a precision resistance, a plurality of on-chip switchable resistors and a resistance comparison unit. The plurality of on-chip switchable resistors are each capable being switched so as to be coupled in parallel to a nominal resistor. The resistance comparison unit compares a plurality of on-chip comparison resistors to the off-chip resistor and couples a number of the on-chip switchable resistors to the nominal resistor so that the combined parallel resistance of the nominal resistor and the on-chip switchable resistors coupled to the nominal resistor is within a predetermined range of the precision resistance.
This resistance circuit offers the advantage of being able to tune an on-chip resistor to a resistance within a predetermined range after the chip has been manufactured.
These and other aspects of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the following drawings. As would be obvious to one skilled in the art, many variations and modifications of the invention may be effected without departing from the spirit and scope of the novel concepts of the disclosure.